1. Field of the Invention
This invention relates generally to drilling machines that provide compressed air to an drill bit.
2. Description of the Related Art
There are many different types of drilling machines for drilling through a formation. Some of these drilling machines are mobile and others are stationary. Some examples of mobile and stationary drilling machines are disclosed in U.S. Pat. Nos. 3,245,180, 3,692,123, 3,708,024, 3,778,940, 3,815,690, 3,833,072, 3,905,168, 3,968,845, 3,992,831, 4,020,909, 4,595,065, 5,988,299, 6,672,410, 6,675,915, 7,325,634, 7,347,285 and 7,413,036. Some drilling machines, such as the one disclosed in U.S. Pat. No. 4,295,758, are designed to float and are useful for ocean drilling. The contents of all of these cited U.S. Patents are incorporated by reference as though fully set forth herein.
A typical mobile drilling machine includes a vehicle and tower, wherein the tower carries a rotary head and drill string. In operation, the drill string is driven into the formation by the rotary head. In this way, the drilling machine drills through the formation. More information about drilling machines, and how they operate, can be found in the above-identified references.
The drilling machine typically includes a power pack, which includes a compressor operatively coupled to a prime mover. The prime mover can be of many different types, such as a diesel engine, gas engine, compressed natural gas (CNG) engine or electric motor. The prime mover provides power to the compressor, and the compressor operates in response. During operation, the compressor provides compressed air to the drill bit through the rotary head and drill string. The compressed air is used to flush cuttings from the borehole.
There are several problems, however, when powering the compressor with the prime mover. For example, the prime mover consumes a significant amount of energy in response to providing power to the compressor. For example, a prime mover which includes a diesel engine consumes a significant amount of diesel fuel in response to providing power to the compressor. A prime mover which includes a gas engine consumes a significant amount of gas in response to providing power to the compressor. A prime mover which includes a CNG engine consumes a significant amount of natural gas in response to providing power to the compressor. Further, a prime mover which includes an electric motor consumes a significant amount of electrical power in response to providing power to the compressor. The energy consumed by the prime mover is wasted if the prime mover provides power to the compressor, but the compressor does not provide compressed air to the drill bit. The compressor is often said to be in standby-mode when it is receiving power from the prime mover and not providing compressed air to the drill bit. It is desirable to reduce the amount of energy consumed by the prime mover in response to the compressor being in standby-mode.
In some situations, the compressor consumes about 25% to about 50% of its maximum rated power in standby-mode. Some compressors included with drilling machines have maximum rated power of between about 200 horsepower to about 600 horsepower. Hence, in standby-mode, the compressor can be consuming about 50 horsepower (25% of 200 horsepower) to about 300 horsepower (50% of 600 horsepower) when compressed air is not being provided to the drill bit. In a typical drilling operation, the compressor is in standby-mode for about 50% of the time. Hence, a significant amount of fuel is consumed by the prime mover and wasted by the drilling machine when the compressor is in standby-mode.